How to add custom icon to EnumProperty in add-on
For each enum item an icon can be specified. You just need to edit this part of your code:

# defined globally
easingItems = [
    ("EASE_IN", "Ease In", "", customIcon["MY_EASEIN"].icon_id, 0),
    ("EASE_OUT", "Ease Out", "", customIcon["MY_EASEOUT"].icon_id, 1),
    ("EASE_IN_OUT", "Ease In-Out", "", customIcon["MY_EASEINOUT"].icon_id, 2)]